The present invention relates generally to an imaging optical system, and more particularly to an imaging apparatus comprising an imaging optical system.
There has been an imaging optical system known so far in the art, which uses five lenses to make improvements in optical performance (see Patent Publications 1 and 2). The publications disclose an imaging optical system that comprises an aperture stop for limiting axial light beams, a lens group that is located on an object side with respect to that aperture stop and consists of two lenses, and a lens group that is located on an image side with respect to that aperture stop, wherein a lens system comprises, in order from its object side, a positive single lens, a negative single lens, a cemented doublet of a negative lens and a positive lens, and a positive single lens.
Patent Publication 1: JP(A) 9-133859
Patent Publication 2: JP(A) 2000-180719
With such imaging optical system as disclosed in Patent Publications 1 and 2, however, the thickness (the axial distance from the object side surface of the lens nearest to the object side to the image side surface of the lens nearest to the image side) grows large relative to the back focus. For this reason, intentional use of such an imaging optical system for interchangeable lenses of single-lens reflex cameras will work against size reductions of the whole imaging apparatus including the imaging optical system.
In view of such problems with the prior art, one object of the present invention is to provide an imaging optical system that works for making sure the given back focus and reducing thickness, and makes it easy to ensure brightness and optical performance. Another object is to provide an imaging apparatus comprising such an imaging optical system.